Unfortunately there are no standard sized aquariums with those dimensions. Does it look commercially built?
https://www.petsolutions.com/C/Aquar...VM0aAsN18P8HAQ
This site has a 20 x 18 glass lid for 30 bucks.
Marineland Perfecto Glass Canopy 20 in. x 18 in.
This site has one for 22 bucks.
https://catalog.elmersaquarium.com/d...ariums-20.html
The dimensions are for what is known as a column aquarium.
This tank is probably gonna be limited to tree frogs just simply due to the limitations of the lids available unless you feel you can build a custom secure screen lid. You could probably modify the lid to allow small arboreal UV required lizards in the tank by maybe cutting out a section of the glass lid in the back and replacing it with screen siliconed in place so the UV doesn't get messed up by the glass.
This tank has natural planted vivarium written all over it.